Fix DBUS Name For BootSource Cd

Commit 9f16b2c1b6b6884482bca304fcaa573caf91d0d6
changed the mapping for redfish boot source 'Cd' to
'xyz.openbmc_project.Control.Boot.Source.Sources.DVD'.
However, that mapping does not match what we have in
phosphor-dbus-interfaces:
'xyz.openbmc_project.Control.Boot.Source.Sources.ExternalMedia'

This commit reverts that mapping back to ExternalMedia.

Tested:
-- Ran redfish service validator - no errors seen.
-- Tested PATCH on BootSourceOverrideTarget with value "Cd"
curl -k -H "X-Auth-Token: $bmc_token" -X PATCH
https://${bmc}:${port}/redfish/v1/Systems/system -d '{"Boot":
{"BootSourceOverrideEnabled": "Continuous", "BootSourceOverrideTarget": "Cd"}}'
{
  "@Message.ExtendedInfo": [
    {
      "@odata.type": "/redfish/v1/$metadata#Message.v1_0_0.Message",
      "Message": "Successfully Completed Request",
      "MessageArgs": [],
      "MessageId": "Base.1.4.0.Success",
      "Resolution": "None",
      "Severity": "OK"
    }
  ]

curl -k -H "X-Auth-Token: $bmc_token" -X GET
https://${bmc}:${port}/redfish/v1/Systems/system
{
  "@odata.context": "/redfish/v1/$metadata#ComputerSystem.ComputerSystem",
  "@odata.id": "/redfish/v1/Systems/system",
....
....
....
  "Boot": {
    "BootSourceOverrideEnabled": "Continuous",
    "BootSourceOverrideMode": "Legacy",
    "BootSourceOverrideTarget": "Cd",
    "BootSourceOverrideTarget@Redfish.AllowableValues": [
      "None",
      "Pxe",
      "Hdd",
      "Cd",
      "Diags",
      "BiosSetup"
    ]
  },

Change-Id: Ibd1fe3bc2c98dc97422b8dd71f9f95f6e9e25285
Signed-off-by: Santosh Puranik <santosh.puranik@in.ibm.com>
diff --git a/redfish-core/lib/systems.hpp b/redfish-core/lib/systems.hpp
index 70e9dac..4544af6 100644
--- a/redfish-core/lib/systems.hpp
+++ b/redfish-core/lib/systems.hpp
@@ -468,7 +468,7 @@
         return "Hdd";
     }
     else if (dbusSource ==
-             "xyz.openbmc_project.Control.Boot.Source.Sources.DVD")
+             "xyz.openbmc_project.Control.Boot.Source.Sources.ExternalMedia")
     {
         return "Cd";
     }
@@ -536,7 +536,7 @@
     }
     else if (rfSource == "Cd")
     {
-        return "xyz.openbmc_project.Control.Boot.Source.Sources.DVD";
+        return "xyz.openbmc_project.Control.Boot.Source.Sources.ExternalMedia";
     }
     else if (rfSource == "Pxe")
     {